Dual slot wideband antenna



July 1, 1958 N. E. LINDENBLAD DUAL SLOT WIDEBAND ANTENNA 2 Sheets-Sheet 1 IN V EN TOR.

Original Filed Sept. 27, 1950 v Ag 5:1. was/vam 4 r rmeA/E Y INVENTOR July 1, 1958 N. E. LINDENBLAD DUAL SLOT WIDEBAND ANTENNA 2 Sheets-Sheet 2 Original Filed Sept. 27, 1950 F .12.

44a 470 400 4.90 500 510 520 530 540 i E L/NDENBLAD FEEQl/E/VCY m/ 44% ice DUAL sror winnnam) ANTENNA Nils E. Lindenhlad, Princeton, N. J., assignor, by mesne assignments, to the United States of America as rep resented by the Secretary of the Navy Original application September 27, 1950, Serial No. 186,969, now Patent No. 2,791,769, dated May 7, 1957. Divided and this application December 21, 1956, Serial No. 629,869

Claims. (Cl. 343-767) The invention relates to antennas and particularly to a slot antenna having an operating band width which is greater than that obtained heretofore in slot antennas, and is a division of my copending application Serial No. 186,969, filed Sept. 27, 1950 now Patent Number 2,791,769, issued May 7, 1957.

The slot antenna is well known in the art, typical examples being shown and described in U. S. Patents 2,414,266 to N. E. Lindenblad, 2,425,303 to P. S. Carter and 2,487,622 to S. Wehner. Such antennas have been adapted for certain uses, perhaps the most important class of which are those intended for installation on high-speed aircraft. There remain, however, a considerable number of applications in which the construction of a slot antenna obviously would be quite advantageous but certain electrical or mechanical limitations preclude the use thereof. The most serious limitation encountered in slot antennas is the relatively limited frequency band width obtainable with prior art arrangements. Numerous suggestions have been made toward increasing the frequency band width of the known slot antennas. However, all of them suffer detriment either to stability as exemplified by variable directive characteristics in such cases where the radiating orifices are subject to relative phase differences or else in an economical sense, and at best have afforded only a relatively small increase in band width.

It is an object of the invention to provide an antenna having the advantageous construction of the known slot antenna and an effectively wider frequency band.

It is another object of the invention to provide a slot antenna having the feed point thereof so located that complernentary inductive and capacitive reactance components in conjunction with apportioning of the radiation resistance component are connected to the associated transmission network to present a substantially constant impedance characteristic over a wide frequency band.

lit is a further object of the invention to provide an antenna having wide frequency band characteristics and especially suitable for installation in restricted space, such as the wing tips of aircraft and the like.

Another object of the invention is to provide an antenna having a substantially constant directivity characteristic over a wide frequency hand.

These and other objects of the invention which will appear as the specification progresses are attained in a receiving or transmitting antenna according to the invention by providing at least one conductive element having two closely parallel unequal length elongated apertures associated therewith, the frequency, and consequently the wavelength, relationship with respect to the length of the apertures being such that the shorter aperture exhibits inductive reactance while the longer aperture exhibits capacitive reactance at the desired operating frequency. The conductors of a two-conductor transmission line are connected individually across the edges of the apertures. in the preferred embodiments, the apertures are in the form of elongated slots and a coaxial transmission line is arranged between the slots with the central conductor thereof connected across the longer slot and the sheath conductor connected across the shorter slot or vice versa, the return path to ground passing from the outer conduc tor of the coaxial transmission line thru the intermediary of a quarter wavelength balun sleeve. The input impedance of the antenna thus obtained is substantially con stant over a very wide frequency band.

The invention will be more completely understood by reference to the following description and the accompanying drawing forming part of the specification and in which:

Figure 1 is a basic arrangement of an antenna according to the invention;

Figure 2 is a cross-sectional view of the antenna illustrated in Figure 1;

Figure 3 is an alternate embodiment of an antenna according to the invention;

Figure 4 is a cross-sectional view of the antenna illustrated in Figure 3;

Figure 5 illustrates the application of the basic antenna of Figure 1 to installation in an aircraft wing;

Figure 6 illustrates how the fundamental radiator illustrated in Figure 1 or Figure 3 may be applied to a tower antenna;

Figure 7 is a cross-sectional view of the antenna illustrated in Figure 6;

Figures 8 and 9 are detailed illustrations of the construction of a slot antenna according to Figure 6;

Figures 10 and 11 are illustrations of alternate embodiments of the arrangement shown in Figures 8 and 9;

Figures 12 and 13 are illustrations of the application of the basic arrangement shown in Figure 1 to circular slot antennas;

Figure 14 is an illustration of the alternate feed system of Figure 3 applied to the circular slot antenna of Figures 12 and 13; and

Figure 15 is a graphical representation of the perform ance of an antenna according to the arrangement shown in Figures 1 and 2.

Referring .to Figures 1 and 2, there are shown elevation and cross-sectional views respectively of an antenna structure according to the basic principles of the invention. This particular embodiment of the invention is intended for installation on a conductive surface element 20, which in practice might be constituted by the external surface of an airplane or other vehicle. Theoretically the conductive surface element extends to infinity but practically there are finite dimensions for each application which will provide the desired results. Such a finite dimensioned element is sometimes termed a semi-infinite element, although the term is not widely used. Two hollow cylindrical conductors 22 and 24 of unequal length are arranged above element 2t) by means of conductive members 26 and 28. Conductors 22 and 24 are supported by members 26 and 28 with the longer conductor 24 above conductor 22. The ends of conductors 22, 24- are electrically interconnected by means of members 26, 28. fact are preferably hollow since high frequency currents are predominant near the outer surfaces thereof. Memers 26, 2% also serve to space conductors 22, 24 from member 26 and each other to provide elongated apertures 31 and 32 of unequal dimensioning. For connecting the desired utilization apparatus, a coaxial transmission line 34 having a central conductor 35 and a sheath 36 is led into the antennas as shown. The central conductor 35 of transmission line 34 extends into conductor 22, which in effect then becomes the sheath conductor of the transmission line. Conductor 35 is led thru an aperture 37 in conductor 22 to a tapering connector element 38 electrically connected to conductor 24. These connections of transmission line 34 to conductors The latter members need not be solid and in 22, 24 are preferably made at the center as shown, but it may be desirable or even necessary to connect at points off center in order to properly match impedances. The impedance values of the arrangement shown are maximum at the center of and tend toward zero at the ends of the slots 31 and 32. It should be understood that it is within the scope of the invention to arrange a compensating stub transmission line within conductor 24 and connect the center conductor of the stub transmission line to conductor 35. The length of the stub together with the type of termination, i. e., open or short circuited, will determine the coupling to slot 32. This type of coupling is described in Patent No. 2,573,460 issued to me on Oct. 30, 1951. Aperture 31 formed by element and conductor 22 is dimensioned with respect to the operating frequency band to provide an inductive reactance component as compared to a capacitive reactance component provided by aperture 32 existing between conductors 22 and 24. Optimum results of this band widening effect are obtained when the inductive and capacitive components have substantially equal portions of the radiation resistance component combined therewith at the midband frequency and when the radiation resistance component is equal to the square root of the inductance and capacity quotient.

A streamlined version of the basic arrangement may be constructed as illustrated in Figures 3 and 4, in which an alternate feed system is also illustrated. A thick, hollow fin member 40 is arranged in conjunction with conductive surface element 20 to provide two slots 41, 42 of unequal length. The frequency, and consequenly the wavelength, relationship to the length of these slots is such that across the inner and shorter slot the impedance is inductive while it is capacitive across the outer and longer slot. The height, cross section and slot lengths are designed to provide the best approach possible to the condition R= /L/ C where R is the radiation resistance, L is the inductive reactance and C is the capacitive reactance of the arrangement. 34 is now snaked thru, inside the intermediate member 48 between the slots of fin member 49. Center conductor 35 is led thru an aperture 43 and connected across long slot 42 to what may then be called the outer and capacitive member. The extremity of outer conductor 36 is connected by a lead 45 which is led thru an aperture 47 and connected across slot 41 to member 20. The return path to ground for the current coming thru outer conductor 36 is coupled to the intermediate conductor 48 at the edges of apertures 43 and 47 by means of a balun sleeve 49 which surrounds the end of transmission line 34 for a distance of substantially a quarter wavelength at the midband operating frequency and is connected to the sheath conductor 36 at the end of the sleeve remote from the end of transmission line 34. Center conductor 35 is then connected to a radiating portion of the antenna which is capacitive, while sheath conductor 36 is connected to a radiating portion which is inductive. Member may be solid except for provisions permitting transmission line 34 to be led thru. However, in the interest of reducing the cost of manufacture and the weight of the finished article, member 46} is preferably made in the form of a hollow casting, and intermediate conductor 48 of member 40 may be arranged to serve as balun sleeve 49 if desired. Member 40 is preferably streamlined especially if the antenna is to be employed on aircraft. However, the streamlining is advantageous not only for aerodynamic reasons but for electrical reasons as well. The current distribution contours of the currents induced in the surface of member 40 will be more uniformly distributed in a streamlined version as shown in Figure 3 than with the rectangular form shown in Figure 1.

It should be noted that an arrangement such as shown in Figure 3 may be streamlined in the direction axially of the slots or perpendicular to the plane of the slots as dictated by the requi ed installation on the aircraft. In

The coaxial transmission line 1 this manner, a highly efficient antenna with low aerodynamic drag can be evolved. The principles of the invention are not confined to an externally mounted protuberant structure, however, but may be most advantageously applied to flush mounting on aircraft and other vehicular external surfaces and the like. Apertures may be set flush with the outer surface of a vehicle structure such as in the arrangements shown in Figure 5 wherein slot antennas 52, and 57 according to the invention are installed in the wing surfaces 51 and 53 of aircraft with the slots lying in the surface of the wing. For slots situated near the edge of the wing, within, for example, one-eighth wavelength, the feed arrangement shown in connection with Figure 1 may be used to advantage. Where, however, the slots are located centrally of a large area of conductive sheeting, the balanced feed arrangement, as described in connection with Figure 3, must be employed. Such an arrangement is by no means limited to aircraft structure either. Referring to Figures 6 and 7, there is shown an adaptation of the basic structure of the invention to a broadcasting antenna system comprising a tower 60 having four radially arranged hollow conductive wing portions 61-64. Each of these wing portions has three sets of wide-band slots 65-66 therein arranged one set above another and energized as described in connection with either of the arrangements shown in Figures l-4. Such a construction may be made with'three or more wing portions. For example, a tower with four wing portions could be used in television broadcasting with the slot antennas of each section connected in parallel and with the sections of antennas operating in phase quadrature as described in U. S. Patent 2,239,724 issued April 29, 1941 to Nils E. Lindenblad. An antenna system using three wing portions may be operated threephase as outlined in U. S. Patent 2,245,693 issued June 17, 1941 to'Nils E. Lindenblad. Other modes of operation will be suggested to those skilled in the art. In any event, there will be a further broadening of the constant impedance frequency band width due to line compensation which occurs in such combinations.

The slots of the antenna according to the invention are preferably spaced at a distance substantially less than one-eighth of the operating wavelength. The closer together the better so long as the conductive material between slots is sufficient to carry the induced currents. Obviously, ample room must be permitted for practicable types of transmission line connections and so forth.

Where the slots are recessed in the surface of a vehicle or antenna tower structure, they are preferably backed with a cavity resonator in order to provide the inductive and capacitive reactance components which are afforded by the use of the large conductors of the basic embodiments. Referring to Figures 8 and 9, there is shown an arrangement with a long slot 82 and a short slot 84 arranged in a conductive surface element 86 with a single structure 86 backing both slots 82 and 84 constituting a cavity resonator in conjunction therewith. A coaxial transmission line 88 having a balun sleeve 89 is led into the cavity resonator between the slots and connected across the slots at or near to the center thereof as described in connection with the arrangement of Figures 3 and 4. A more detailed description of the balun shown and other practical forms of baluns may be had by reference to U. S. Patent 2,231,839 to Nils E. Lindenblad.

Obviously, the split sheath conductor type of balun described in U. S. Patent 2,454,907 to George H. Brown may also be adapted to the purpose. Alternatively, as shown in Figures 10 and 11, each of the slots may be backed by individual structures 92 and 94 constituting separate cavity resonators, and transmission line 88 is again connected across slots 81, 82 at or near the center thereof. The latter arrangement is to be preferred since tuning of the individual components is etfected by tuning the cavity in known manner, whereas in the former arrangement the slots dimensions must be varied.

It should be understood that the slots of an antenna structure according to the invention may either be open or covered with dielectric material or even completely filled with a suitable dielectric material, the change in characteristics of the antenna being taken into account in the design of the slots and covers.

Referring to Figures 12 and 13, there is shown an adaptation of the principles of the invention to the ringshaped slot antenna. Whether a ring-shaped slot antenna is fed in phase quadrature as shown in the drawing or in opposition as in other applications, it can be greatly improved by employing two concentric ring slots 122, 124 backed by conductive surface structures 131 and 133 constituting members forming cavity resonator elements in conjunction with conductive surface element 120. The resultant cavity resonators are of such dimensions that one slot presents inductive reactance and the other capacitive reactance. Either the inner or outer slot may be made inductive or capacitive as desired. However, it will be obvious that a simpler and more advantageous arrangement is had when the inner slot is made inductive. If both slots have about the same radiation resistance and if the inductance and capacity values are so proportioned that the radiation resistance of each slot equals the square root of the inductance-capacity quotient, an approach toward constant impedance over the entire frequency band will be obtained.

In practice, a wide band ring-shaped slot antenna is preferably formed by a drum member having a circular portion 141, a cylindrical wall portion 143, and an annular lip portion 145, leaving a central aperture in which a circular plate member 147 is arranged on a support member 149 fastened to plate portion 141. Plate member 147 is smaller in diameter than the inner diameter of lip member 145, thus leaving an annular slot 122. Slot 122 of cavity resonator 131 is set flush with the conductive surface element 120 by means of another circular base portion 151 to which support 149 is aflixed and which is fastened to element 120 by means of a wall portion 153. Slot 124 is formed between wall portions 143 and 153, between which wall portions and plate portions 141 and 151 a second cavity resonator is formed.

The outer cavity is coupled to a plurality of transmission lines 161 having a sheath conductor 163 connected to wall portion 153 and a central conductor 164 connected to wall portion 143 as shown. Variations in the coupling of course should be obvious to one skilled in the art. For example, the degree of coupling may be varied by moving the points of connection along wall member 143 and 153 parallel to the axis of the drum members 131, 133, or the axis of support 149. Also, the connecting points may be found on members 141 and 151 depending on the impedance level to be matched and the desired mode of operation.

An alternate feed arrangement, shown in Figure 14, employs the balanced connection afforded by the incorporation of a balun sleeve 169 having a length of the order of a quarter wavelength at the midband frequency. The end of balun sleeve 169 remote from the end of transmission line 165 is connected to sheath conductor 167, thereby isolating the end of sheath conductor 167 from ground in known manner. Balanced feed connections from the ends of conductors 166, 167 then connect to the edegs of slots 122 and 124. The transmission line is preferably arranged inside wall 143 and below wall 141, in which manner the cavity structures are equally affected by the inclusion of balun sleeve 169. Transmission line 165 is then lead out of structure 133 near outer part 149 in order to minimize any deleterious effects due to the presence of the line within the cavity structure.

A graphical representation of the results obtained with an antenna according to the invention is shown in Figure 15. Standing wave voltage ratio is plotted against the operating frequency for different values of slot length ratio. Curves 171, 172 and 173 were obtained from a rough model constructed along the lines: of the arrangement shown in Figures 1 and 2. The interesting part of the curves is below the 21 standing wave ratio line, since that figure is usually accepted as good tolerance. The other arrangements, however, will provide results at least as good as those indicated in the graph.

The radiation pattern obtained with the antenna according to the invention exhibits substantially constant directivity over a wide frequency band since the spacing between slots is substantially less than one-eighth wavelength, thereby avoiding any combining elfect such as found in arrays of radiator or receptor elements.

What is claimed is:

1. An antenna system including an elongated hollow metallic structure having a cross-section comprising at least three hollow wing portions extending from the central axis of said structure, each of said wing portions having at least one set of elongated apertures therein lying side by side, one of said apertures being longer than the other, and means to couple a transmission line to said structure at each set of apertures to energize the same.

2. An antenna system including an elongated hollow metallic structure having a cross-section in the form of a cross, said structure having hollow wing portions emanating from the central longitudinal axis of said structure, each of said wing portions having at least one pair of elongated slots therein lying side by side, one of the slots of each pair being longer than the other, and means to couple a transmission line to each pair of slots.

3. An antenna system including an elongated hollow metallic structure having a cross-section comprising a plurality of hollow wing portions emanating from the central axis of said structure, each of said wing portions having at least one pair of unequal length elongated slots arranged therein in parallel relationship to each other and to said central axis, and means to couple a two-conductor transmission line to each. pair of slots, the connections of said transmission line conductors being made individually across opposing edges of the longer of said slots.

4. An antenna system including a number N of radiating elements disposed radially about a central axis, each of said elements being spaced substantially 360/N degrees from adjacent radiator elements, each of said radiator elements comprising a conductive surface element having at least one set of elongated slots therein in parallel relationship to said axis, said slots having dimensions at which the product of the reactance value of the slots of each set is substantially equal to the square of the radiation resistance value of each component of said antenna, and means to maintain radio frequency potential across said slots.

5. An antenna system including an elongated hollow metallic structure having a cross-section in the form of a cross, said structure having hollow wing portions emanating from the central longitudinal axis of said structure, each of said hollow wing portions having at least two pairs of elongated slots therein, said pairs of slots being in registry, the slots of each pair lying side by side, one of the slots of each pair being longer than the other, and means to couple a transmission line to each pair of slots.

No references cited. 

